1. Field
The invention relates to television color cameras, and particularly to a hybrid color camera configuration employing a camera pickup tube in combination with a selected number of solid state sensors.
2. Prior Art
At present, practical color cameras can be classified into several basic types. A first type employs one or two camera tubes and color analysis is performed by a rotating disc containing red, green and blue, or red and blue, filters interposed between the camera optical lens and the camera tube. As the disc rotates, successive filters intercept the light directed to the tube target to generate signals corresponding to the red, blue (and green) color components of the scene. A typical camera is described in "New Two-Tube Color Cameras For Broadcast Use", B. M. Poole, Tech. Papers, NAB Eng. Conf., Mar. 23-26, 1969, pgs. 238-244.
In another type of camera, one tube is used and each point of the optical image is split into, for example, three color components by interposing an array of focused fine color filter stripes in the optical path. Each image point produces charged target areas corresponding to the color component points which are scanned by the tube electron beam, to generate a signal which sequentially describes the area charges. Electrical processing derives signals corresponding to the color components. Typical of such a one-tube camera is that described in the article "A Single Vidicon TV Camera System", Journal of the SMPTE, Vol. 79, April 1970, pgs. 326-330.
Other types of cameras employ two, three or four camera tubes, wherein the color components of the optical image are formed separately and the related signals generated continuously. The light from the camera lens is split into red, green and/or blue color components via dichroic mirrors, where each component forms an image on the camera tube target. Typical three and four tube cameras are described in; Ampex Corp. Catalog No. 1809173-01, "BC-230B, Manual of Theory of Operation", June 1975; Cohu Electronics, Inc., "Operating and Maintenance Instructions For 9800 Series Color Video Encoder", June 1970; RCA Catalog B.2000, "Transistorized Live Color Camera, Type TK-42", 1965; Sound and Vision Broadcasting, "A New Four-Tube Colour TV Camera", Vol. 7, No. 1, Spring 1966, pgs. 8-21.
In multiple tube cameras, the images formed on the targets must have correct spatial correspondence relative to each other to ensure that the color component pictures at the display are in register; i.e., spatially superimposed at all points. Consequently, the color analysis arrangements and the camera tubes must be very stable mechanically, and the scanning patterns traced on the tube targets must be as stable and identical as possible.
In any camera, the characteristics of a camera tube that relates output signal and scene brightness together with that of any gamma corrector used, must be such that over a large range of scene brightness, an approximately linear relationship exists between a change of scene brightness and the corresponding change at the display. Therefore, in a camera incorporating several tubes (and several gamma correctors), the combined characteristics of one tube and its associated corrector must be well matched to the others. Thus the deflection assemblies for the pickup tubes are first computer-matched to provide yoke and tube combinations of similar characteristics, and thus scan raster geometries which are as similar as possible. In addition, one tube (for example, the green-channel tube) may be selected as a master tube/channel, and various analog-waveform driving signals are applied via suitable electronics to match the scan rasters of the remaining slave tube, or tubes, of corresponding slave channels, with the master tube raster, as generally described in the multiple tube references of previous mention.
None of the above techniques are capable of providing the degree of accuracy required to cause a pickup tube to generate a scan raster with the near perfect physical geometry of a solid state image sensor. In addition, most of the multiple tube cameras are bulky and heavy, require skilled operation, while lacking sensitivity and economy of operation.
Recent developments in the state-of-the-art of solid-state sensors have spurred interest in their use in relatively low resolution electronic news gathering (ENG) color cameras in place of the conventional Plumbicon, Vidicon, Saticon, etc., pickup tubes. By way of example, typical ENG color cameras employing solid state sensors are described in; "All-Solid-State Camera for the 525-Line Television Format", IEEE Transactions on Electron Devices, Vol. ED-23, No. 2 February 1976, pgs. 183-189; "Solid State Image Sensors: Improvements in Signal Processing Techniques", BBC Research Dept., Report BBC RD 1976/4, January 1976; RCA Brochure on the SID 51232, printed Jan. 1975. However, a typical broadcast quality television color camera system requires a luminance component of 4.2 megaHertz bandwidth or better. This parameter imposes the requirement that a solid state array have a geometric configuration of the order of 600.times.488 photosensitive elements. However, the geometry of presently available state-of-the-art solid state sensor arrays, and those in the foreseeable future, provide only on the order of 320 by 244 photosensitive elements. Typical thereof, a photosensitive element of one type of such an array may contain 2 MOS transistors, 2 capacitors and 2 reverse biased photo diodes, whereby it is very difficult to accurately manufacture arrays with the greater number of elements required for broadcast quality color television use. It is estimated that several years development time is needed before the advent of such high resolution solid state sensors. Thus, the development of an RGB color camera of broadcast quality utilizing solid state devices alone is improbable in the immediate future.
However, an array with as few as 188 by 244 elements offers sufficient resolution to allow generation of the red, green and blue chroma signals, or of the R-Y and B-Y color components, since the chroma signals require on the order of 1 megaHertz bandwidth, or less. Accordingly, the specific combination of a pickup tube with presently available solid state sensors having a relatively small number of photosensitive elements, allows the development of a light weight, broadcast quality color camera of improved configuration.
When utilizing a pickup tube with solid state sensor devices, problems in registration arise due to the difference between the near perfect array geometry of the accurately manufactured solid state devices and the relatively inaccurate scan raster of the tube, which cause, in turn, loss of resolution, color edge effects, etc. Thus, means must be provided to correct the tube scan raster to precisely conform same to the near perfect array geometry of the solid state sensors. This presents a different problem than that of prior art multiple-tube cameras as, for example, the four tube camera, wherein the various tube scan rasters need only be matched to each other, not made to conform to near-perfect array geometry.